The invention relates to a diesel exhaust filter. Specifically the invention relates to a mullite aluminum titanate diesel exhaust filter and a method of making the same.
In the industry cordierite (2MgO.2Al2O3.5SiO2) has been the cost-effective material of choice for diesel exhaust aftertreatment applications, such as wall-flow filters, due to its combination of good thermal shock resistance, filtration efficiency, and durability under most operating conditions. However, under certain circumstances cordierite filters are susceptible to damage and have even catastrophically failed. Occasional thermal runaway occurs during uncontrolled regenerations resulting in localized cordierite melting. Another factor contributing to failure occurs when metal impurities from the engine oil, catalytic additives or corroded metal from the exhaust port liners are introduced into the filter during operation. Typically, at temperatures exceeding 1300° C., these metals form oxides which react with the cordierite structure. Evidence of the failed material are usually small holes on the filter where the metal initially deposits and reacts causing corrosion and melting of the material.
Recently, silicon carbide (SiC) wall-flow filters have been made commercially available for diesel exhaust filtration. However, SiC filters are costly to manufacture, and must be segmented due to an inherent high coefficient of thermal expansion (CTE) and poor thermal shock resistance.
Aluminum titanates, in particular mullite aluminum titanate, have been proposed for usage as filters, in particular, as diesel particulate filters in U.S. Pat. Nos. 4,483,944, 4,767,731, 4,855,265, 5,290,739 and EP 0 036 462 and EP 0 873 775. Such bodies, however, are expected to have low permeability, and consequently high pressure drop or back pressure against the engine, due to low porosity or fine pore size; or in other cases, to have porosities and pore sizes so large that the bodies would be expected to exhibit poor filtration efficiency in removal of particles from a gas stream.
A need therefore exists to have a mullite aluminum titnate diesel particulate filter with higher permeability than has been achieved up to now, while at the same time with useful filtration efficiency, low thermal expansion, high heat capacity, high thermal shock resistance, high mechanical strength, and increased durability during thermal cycling.